1. Field of the Invention
The present invention relates generally to thermal energy storage materials and, more particularly, to phase change materials capable of storing large amounts of energy per unit of volume. Specifically, the present invention relates to a novel phase change thermal energy storage composition which has characteristics capable of efficient utilization in a variety of thermal energy storage environments as well as in certain medical applications.
2. Description of the Prior Art
Energy systems utilizing alternate fuel sources have been studied intensely in recent years. As a part of such studies research has been conducted toward processes and mechanisms designed to store thermal energy generated from a wide variety of sources. One particular area presently under investigation deals with the storage of thermal energy created by solar energy systems, and in particular increasing the efficiencies of such storage of thermal energy.
A wide variety of liquid and solid materials have been utilized to store thermal energy. Typically, water, glycol, oil and the like have been utilized as liquid media for transferring and/or storing thermal energy in certain solar energy systems. In addition, solid materials such as concrete, brick and the like have also been utilized in passive solar systems for storing thermal energy. One example of such is the Trombe wall wherein direct solar gain on one side thereof is stored and reradiated into living areas from the opposite side thereof. Such storage mediums, however, have been limited in their per unit volume capability of storing thermal energy as well as their capability of storage for prolonged periods to provide a delay time in energy delivery.
During the process of investigating candidate materials for thermal energy storage, it was discovered that phase change materials, both solid-solid as well as solid-liquid, have the capability of storing exceptionally large amounts of energy per unit volume at temperatures above their phase change temperature. Appropriate applications of such phase change materials are dependent on the actual transition temperature of the particular material. In buildings utilizing passive solar energy systems, thermal energy storage is needed to prevent over-heating during daylight hours and to prolong the delivery of solar heat into the night. Traditionally, massive materials of construction such as masonry, adobe, or water in storage tanks have been used to store solar heat. While these methods are effective and have been successfully used in many building applications, they do not fit well into the growing trend of light-weight and factory-built homes.
In energy conserving commercial buildings, there are opportunities for using thermal energy storage to reduce peak cooling power demands, to reduce chiller equipment size and cost, and to improve the efficiency of air conditioning system performance. Thermal energy storage materials may be used as a heat sink during the day to absorb internally generated heat and maintain comfort. At night, the stored heat is removed by forced air ventilation or the use of compression chillers operating at the lower ambient air reject temperature and at the lower off-peak utility rate. Phase change thermal energy storage materials would be highly useful for these and a variety of other purposes.
Phase change thermal energy storage materials have been utilized in a variety of energy system applications in the past. Crystalline polymers have been utilized as phase change materials for heat storage as disclosed in U.S. Pat. Nos. 4,063,546, 4,149,016 and 4,176,655. Mixtures of non-hydrated materials have also been used as phase change thermal energy materials such as disclosed in U.S. Pat. Nos. 4,309,297 and 4,470,917. Moreover, crystalline polymer compositions as well as halocarbon based materials typical of those used in refrigeration systems have also been used as thermal energy storage compositions. Examples disclosing such materials are illustrated in U.S. Pat. Nos. 4,182,398, 4,259,198, 4,473,484, 4,157,976, 4,455,247 and 4,468,337. Finally, the most efficient solid-liquid phase change thermal energy materials utilized to data include the halide salts such as disclosed in U.S. Pat. Nos. 3,958,101, 3,976,584, 4,081,256, 4,209,513, 4,273,667, 4,277,357 and 4,280,553. There are, however, distinct problems with such halide salts. They are corrosive and damaging to concrete and wood if container failure occurs, and containers must also be impervious to water vapor to prevent a change in composition and loss of the reversible phase transition. Moreover, these salts may be a health hazard if the container leaks. Some of these salts suffer from cyclic instability. Thus, there remains a need for a solid-liquid phase change thermal energy storage material which overcomes the aforementioned problems, is inexpensive to manufacture and is stable for long term usage.